Seals are used to prevent or minimize leakage of a fluid through mechanical clearances in either the static or dynamic state. In dynamic conditions, a shaft seal is typically placed between a stationary housing and a rotating shaft. Typically, the rotating shaft penetrates a barrier or ring seal. When used in a compressor, a ring seal typically separates a high pressure chamber and a low pressure chamber. Fluid pressure will attempt to escape through any clearance between the shaft and ring seal. However, a clearance must exist between the shaft and seal to allow free rotation of the shaft. Accordingly, the seal must minimize the loss of high pressure fluid through the clearance while not interfering with the rotation of the shaft.
One prior art solution has been the use of windback seals for rotatable shafts in which the seal elements incorporate a plurality of helical threads or grooves to provide a pumping action on the fluid passing through the seal so that a portion of the fluid can be redirected towards the high pressure side of the seal element. The helical grooves can be in the inner cylindrical surface of a stationary seal element or in the external surface of the shaft or a sleeve shrunk onto the shaft. The grooved surface can be opposed by a plain surface or by another grooved surface. These windback seals have been used to prevent migration of sour oil into the machine when fitted at an inner labyrinth, and to prevent cross migration of lubricant oil to seal oil or seal oil to lubricant oil when fitted in a chevron arrangement at a separation labyrinth location. However, in some windback seals the thread grooves extend the length of the seal element, making it more difficult to maintain an adequate seal under static conditions, i.e. when the shaft is not rotating. The use of fine or very shallow depth grooves in a windback seal element has been proposed and would generally provide a better static seal than the deeper grooved windback seal elements, but such shallow grooves are generally more sensitive to mechanical damage resulting from occasional contact between the sealing surface and the radially adjacent surface than the windback seal elements with deeper grooves.
Thus, a need exists for a shaft seal that not only entrains a fluid across the seal as a fluid film, but redirects at least a portion of the fluid back toward the high pressure side of the seal element during the rotation of the shaft, while providing a reasonable degree of sealing under static conditions.